To unlock the triangle inequality which will be used to prove that balls are open, introduce a level before to unlock that theorem. An idea for this is to prove a statement about a logical equivalence related to the triangle inequality (with reverse triangle inequality?)
Presumably first level.
To unlock the triangle inequality which will be used to prove that balls are open, introduce a level before to unlock that theorem. An idea for this is to prove a statement about a logical equivalence related to the triangle inequality (with reverse triangle inequality?)